Spectrum ROM: Routine at 169E

169E: THE 'RESERVE' SUBROUTINE

Used by the routine at BC_SPACES.

On entry here the last value on the machine stack is WORKSP and the value above it is the number of spaces that are to be 'reserved'.

This subroutine always makes 'room' between the existing work space and the calculator stack.

Input
BC	Number of spaces to reserve

Output
DE	Address of the first byte of the new space
HL	Address of the last byte of the new space

RESERVE

169E

LD HL,($5C63)

Fetch the current value of STKBOT and decrement it to get the last location of the work space.

16A1

DEC HL

16A2

CALL MAKE_ROOM

Now make 'BC spaces'.

16A5

INC HL

Point to the first new space and then the second.

16A6

INC HL

16A7

POP BC

Fetch the old value of WORKSP and restore it.

16A8

LD ($5C61),BC

16AC

POP BC

Restore BC - number of spaces.

16AD

EX DE,HL

Switch the pointers.

16AE

INC HL

Make HL point to the first of the displaced bytes.

16AF

RET

Now return.

Note: it can also be considered that the subroutine returns with the DE register pair pointing to a 'first extra byte' and the HL register pair pointing to a 'last extra byte', these extra bytes having been added after the original '(HL)+1' location.